Dr. Alexis Carrel, winner of the Nobel Prize for Physiology or Medicine in 1912, commenting on GKC's book Heretics:

The extreme clarity and brilliance of his style impressed me greatly. The train of his thought appeared to me as strong, flexible, and shining as a steel blade, and as merciless. 

-Chesterton as Seen by His Contemporaries, Cyril Clemens (1939)